Generally, a plug-containing type internally threaded anchor to be set in concrete is known which is used to firmly fix ceiling member, floor members, wall members or other hardware to the surface of a concrete structure by inserting the plug into the hollow inner space of the anchor body, hammering a driving rod to drive in the plug to spread the anchor body from its front end to thereby fixedly embed the anchor in an embedment hole formed in the concrete structure, and finally driving a male screw into the internal thread. For prevention of slip-off of the plug, the prior art resorts to a means shown in FIG. 5.
This will be later described in detail, but here suffice it to say that since the internal thread integrally formed in the anchor body by cutting has an opening with a greater inner diameter than the outer diameter of the plug because of the need to make it possible to insert the plug into the anchor body, there is, of course, the danger of the plug slipping off the anchor body. For this reason, it has been common practice to use grease or an adhesive agent to hold the plug in fixed condition in the anchor.
Whether an adhesive agent is applied to the plug or whether grease is injected into the anchor body after the plug has been inserted into the anchor body, however, in addition to a manual operation required for each anchor body it is necessary to fix the plug in the anchor body and hence not only is there a possibility of omission of application of adhesive agent or grease but also the operation involved is very troublesome, leading to high cost and delayed shipment of the product.
Further, the fixed condition of the plugs inevitably varies in completeness and in accuracy of uniformity. For example, if there is a drying or omission of application of adhesive agent or grease, this can result in plugs slipping off the anchor bodies under vibration, shock or other external forces during transport to the laying site or during laying operation, which slip-off often poses problems decreasing the efficiency of operation, particularly a laying operation which requires the operator to look upward, as in suspending a ceiling member from a concrete structure.
Another known technique is disclosed in Japanese Utility Model Publication No. 3384/1969, which technique, as will be later described with reference to FIGS. 6 and 7, consists in squeezing the anchor body by press operation subsequent to insertion of the plug, so as to form a reduced-diamered neck portion intended to prevent slip-off of the plug.
With this technique, whose problems will be later described in comparison with an embodiment of the present invention, however, the processing steps of forming the internal thread by cutting and of squeezing the anchor body by press operation prior to said thread cutting are indispensable; thus, at least two steps are required in production and corresponding machining equipement has to be prepared; thus, there is a problem that for such mass-consumed machine element, mass-production effect cannot be expected to the fullest extent.